The present invention relates to an input circuit for an AM/FM radio receiver having an AM/FM intermediate frequency of 10.7 MHz, having a circuit for selecting the FM frequency away from the AM frequency at the antenna input, having an FM intermediate frequency rejection circuit at the output of the circuit for selecting the FM frequency away from the AM frequency including a rejection circuit for AM image frequency rejection, having an FM input circuit for reception of frequency-modulated input signals and having an AM input circuit for reception of amplitude-modulated input signals.
Radio receivers having optional AM operation (amplitude modulation) and FM operation (frequency modulation) may have both AM and FM preselector stages downstream from a common preselection filter at the antenna input. The AM preselector stage includes an AM high frequency (HF) selection circuit and the FM preselector stage includes at least one FM HF selection circuit. When the AM and FM preselector stages are connected, both preselector stages must be well isolated from one another to guarantee good reception quality for each reception range. In particular, the AM preselector stage should not represent any significant capacitive load for the FM preselector stage in this regard.
An object of an exemplary embodiment of the present invention is to provide a rejection circuit for each reception range so that the FM input circuit and the AM input circuit remain well isolated from one another with either FM or AM operation selected. FM intermediate frequency immunity is to be increased and AM image frequency immunity is to be expanded.
In an input circuit according to an exemplary embodiment o the present invention, an AM image frequency rejection circuit is connected downstream from the FM intermediate frequency rejection circuit, and PIN diodes for switching from AM to FM operation are provided between the FM intermediate frequency rejection circuit and the AM image frequency rejection circuit, with the PIN diodes connecting the FM intermediate frequency rejection circuit to ground for high-frequency signals when FM operation is selected, whereas in AM operation a control voltage for automatic gain control (AGC) is provided on the switching device and is applied between the FM intermediate frequency rejection circuit and the AM image frequency rejection circuit.
FM and AM preselector stages are operated in alternation with PIN diodes. Due to the HF grounding of the FM intermediate frequency rejection circuit and of the AM image frequency rejection circuit connected to it, the AM preselector stage is uncoupled and the FM intermediate frequency rejection circuit is operated as an acceptor circuit. It is believed that FM intermediate frequency immunity is thus greatly increased.
When the input circuit is switched to AM operation using the throw-over switch, the amplitude-modulated high-frequency signal goes to the AM image frequency rejection circuit, so that interference frequencies of this frequency range upstream from the active AM preselector stage are believed to be greatly reduced in an advantageous manner, and selectivity demand downstream from the AM preselector stage can be reduced. Upstream from the AM image frequency rejection circuit, the amplitude-modulated high-frequency signal is sent through the FM intermediate frequency rejection circuit which reduces FM interference signals through coil L1 of the FM intermediate frequency rejection circuit in AM operation and then functions as a capacitive coupling at its output C1. Due to the fact that the AM image frequency rejection circuit is connected upstream from the active AM preselector stage, AM image frequency immunity is also believed to be greatly improved in an advantageous manner.
The FM intermediate frequency rejection circuit may include a series connection of a coil and a capacitor. In AM operation, the capacitor then assumes the function of capacitive coupling of the AM preselector stage to the preselection filter.
The AM image frequency rejection circuit may include a parallel circuit of a coil and a capacitor and have the function of a frequency band-stop filter.
The FM intermediate frequency rejection circuit and the AM image frequency rejection circuit are connected in series in a particular embodiment, with the PIN diodes connected to the tie point of the series circuit. The first PIN diode is connected to ground in the forward direction, and the second PIN diode is connected to a control voltage in the forward direction. To do so, the anode of the first PIN diode is connected to the tie point of the series circuit, and the cathode of the first PIN diode is connected to ground. The cathode of the second PIN diode is also connected to the tie point of the series circuit, and the anode of the second PIN diode may be connected to the control voltage across a resistor. The anode of the second PIN diode must also be connected to ground via a capacitor.
Commercially available PIN diodes may behave like ordinary diodes at low frequencies, having a low loss and a low junction capacitance. At higher frequencies, PIN diodes may no longer have a rectifier effect, but they may behave like a resistor, whose resistance decreases with an increase in conducting-state current. Then if a constant voltage is applied to the second PIN diode, both PIN diodes have a HF connection in the forward direction. The HF parallel circuit of the very low-resistance equivalent resistors of the PIN diodes connects the FM intermediate frequency rejection circuit directly to ground. Thus the AM preselector stage is completely isolated as soon as FM operation is selected by a constant control voltage applied to the second PIN diode. When a known control voltage is applied to the second PIN diode in AM reception for automatic gain control (AGC), both PIN diodes operate in high-frequency mode and behave like resistors. Then the FM intermediate frequency rejection circuit no longer has high frequency grounding by the parallel circuit of PIN diodes and it behaves only as a capacitive connection having properties for selecting away from the downstream AM image frequency rejection circuit and the AM preselector stage.